1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to millimeter-wave antennas and arrays thereof.
2. Description of the Related Art
As noted by the Institute of Electrical and Electronic Engineers (IEEE): “The millimeter-wave region of the electromagnetic spectrum is usually considered to be the range of wavelengths from 10 millimeters (0.4 inches) to 1 millimeter (0.04 inches). This means they are larger than infrared waves or x-rays, for example, but smaller than radio waves or microwaves. The millimeter-wave region of the electromagnetic spectrum corresponds to radio band frequencies of 30 GHz to 300 GHz and is sometimes called the Extremely High Frequency (EHF) range. The high frequency of millimeters waves as well as their propagation characteristics (that is, the way they change or interact with the atmosphere as they travel) make them useful for a variety of applications including transmitting large amounts of computer data, cellular communications, and radar.” See http://www.ieee-virtual-museum. org/collection/tech.php!id=2345917&lid=1.
In addition, non-lethal directed-energy weapons have recently been developed that use beams of millimeter-wave electromagnetic energy to deter advancing adversaries. In this application, high-power millimeter-wave beams carrying tens to thousands of watts are used to stop, deter and turn back an advancing adversary from a relatively long range.
Prior attempts to produce high-power millimeter-wave beams carrying hundreds or thousands of watts have focused on the use of a single vacuum-electron device such as a traveling-wave tube, a klystron, or a gyrotron as a millimeter-wave source. Systems built around such sources are typically large and heavy, thus limiting the platforms onto which they can be integrated.
Prior attempts to produce millimeter-wave beams with solid-state devices have utilized waveguide, microstrip, and quasi-optical power combining techniques. At millimeter-wave frequencies, waveguide and microstrip power combining typically produce unsatisfactory results due to excessive losses in the waveguide and/or microstrip medium. One current approach involves he use of a reflect array amplifier. The reflect array has independent unit cells, each containing its own input antenna, power amplifier, and output antenna. These unit cells are then configured into an array of arbitrary size. Reflect arrays overcome feed losses by feeding each element via a nearly lossless free-space transmission path. As disclosed and claimed in U.S. Patent Application entitled REFLECTIVE AND TRANSMISSIVE MODE MONOLITHIC MILLIMETER WAVE ARRAY SYSTEM AND IN-LINE AMPLIFIER USING SAME, U.S. application Ser. No. 10/734,445, filed Dec. 12, 2003 by K. Brown et al., the teachings of which are hereby incorporated herein by reference, reflect arrays differ from conventional arrays in that the input signal is delivered to the face of the array via free space, generally from a small horn antenna.
An active reflect array consists of a large number of unit cells arranged in a periodic pattern. Each reflect array element is equipped with two orthogonally-polarized antennas, one for reception and one for transmission. That is, reflect arrays typically receive one linear polarization and radiate the orthogonal polarization, e.g., the receive antenna receives only vertically-polarized radiation and the transmit antenna transmits only horizontally-polarized radiation.
Higher power levels are attained by combining the outputs of multiple transistors. The drawback of this approach is that the power combiners themselves take up valuable area on the semiconductor wafer that could otherwise be occupied by power-generating circuitry.
Consequently, there was a need in the art for an improved system or method for generating a high-power millimeter-wave beam. Specifically, there was a need for a reflect array antenna capable of generating high-power millimeter-wave energy without significant loss.
The need was addressed by copending U.S. patent application Ser. No. 11/508,806 entitled AMPLIFIED PATCH ANTENNA REFLECT ARRAY, filed Aug. 22, 2006 by K. W. Brown the teachings of which are hereby incorporated by reference herein. although this design addressed the need in the art, the array required high current levels due to the parallel orientation of the amplifier columns in the array with respect to the direct current fee thereof. With multiple parallel columns in the array and potentially multiple chips, thousands of amps of current may be required. This requires high current cabling and tends to be lossy. This translates to higher power requirements, higher costs and more bulky arrays.
Hence, a need remained in the art for further improvements to systems and methods for generating high-power millimeter-wave beams. Specifically, a need remained for a reflect array antenna capable of generating high-power millimeter-wave energy with minimal power requirements.
This need was addressed by copending U.S. patent application Ser. No. 11/508,085 entitled SERIES FED AMPLIFIED PATCH ANTENNA REFLECT ARRAY, filed Aug. 22, 2006 by K. W. Brown the teachings of which are hereby incorporated by reference herein.
Millimeter-wave energy is useful for non-lethal directed-energy applications because it penetrates less than 1/64th of an inch into the skin and produces an intense burning sensation that stops when the transmitter is switched off or when the individual moves out of the beam. Realization of this effect requires that the power density exceed a minimum value Pmin.
As disclosed in the above-referenced patents and applications, projection of the minimum required electromagnetic power density over a target area of sufficient size at the desired range requires a sizable transmitter, consisting of a millimeter-wave source, a power supply, a cooling system, and other support equipment. The size and weight of the system are determined primarily by the total radiated power, which in turn is determined by the desired range and the size of the target area to be illuminated.
Conventional systems generate a single beam whose power density is maximal at the center of the target area and decreases monotonically with distance from the center. If it is desired to illuminate a target area of radius ρ0 over which the power density is to exceed Pmin at a distance R from the transmitter, the total radiated power required is that which produces a spot whose power density falls to Pmin at a distance ρ0 from the center. The power density at the center of the target area is typically between one and two times Pmin. As it is difficult to refocus systems of conventional design, targets at ranges r<R cannot in general be optimally illuminated.
Hence, to project the minimum required electromagnetic power density over a spot of sufficient size at the desired ranges by conventional means requires a large transmitter, consisting of a millimeter-wave source, a power supply, a cooling system, etc. The size and weight of such a transmitter limits the platforms capable of supporting such a system. This is a problem that is common to directed-energy systems in general. In the past, this problem was solved by trading increased antenna size for transmitter size and weight reductions. That is, by increasing the size of the antenna to produce more gain, one can achieve the desired power density at range with a smaller transmitter. This trade-off can be carried only so far, since the projected beam of electromagnetic energy shrinks in cross section as the antenna gain increases, reducing the coverage area and putting increased demands on the antenna pointing and tracking accuracy.
In short, conventional millimeter-wave systems of conventional design generate beams having definite power densities at a given range with considerable associated size, weight, cost and power requirements. Further, conventional systems do not allow for the range of the antenna at which power is optimized to be adjusted dynamically.
Hence, a need remains in the art for a millimeter-wave system that offers improved coverage with lower associated size, weight, cost and power requirements.